Particulate aluminum nitride and method for producing thereof

ABSTRACT

A method for producing aluminum nitride includes preparing a slurry containing pseudoboemite and carbon, filtrating and dehydrating the slurry to form a precursor, and calcinating the obtained precursor in an atmosphere containing nitrogen. The slurry is prepared by dispersing a carbon powder in an aqueous solution having a pH adjusted in a range of 8.5 to 9.5, and then synthesizing pseudoboemite by using at least one of an aluminum salt and aluminic acid as an aluminum source, while maintaining the pH of the reaction mixture in a range of 8.5 to 9.5. The aluminum nitrite obtained by the method has a three-dimensional network structure and thus exhibits excellent heat-conducting property and can be suitably used as a filler for an electronic material.

TECHNICAL FIELD

The present invention relates to a method for producing aluminum nitridehaving a three-dimensional network structure by carbothermalreduction-nitridation of a precursor composed of pseudoboehmite having afibrous structure provided with a carbon source in a uniform manner. Inparticular, the present invention relates to a method for producingparticulate aluminum nitride which is suitable for a filler for asemiconductor package molding compound, and the obtained particulatealuminum nitride.

BACKGROUND ART

Those well-known as the method for producing aluminum nitride includethe direct nitridation and the carbothermal reduction-nitridation. Inthe direct nitridation, metal aluminum is reacted in a nitrogenatmosphere to directly produce aluminum nitride. On the other hand, inthe carbothermal reduction-nitridation, an alumina powder is mixed witha carbon powder or the like as a reducing agent, and the mixture isreacted in a nitrogen atmosphere to produce aluminum nitride. In thismethod, the product quality is affected by the particle size and thepurity of the alumina powder as the raw material, it is possible toproduce aluminum nitride having a high purity and a uniform particlesize. Therefore, the carbothermal reduction-nitridation is widely usedas a method for producing aluminum nitride powder as a raw material forsintered body. The sintered body, which is based on the use of thealuminum nitride as described above, has a high thermal conductivity andhigh electrical resistivity. Therefore, the sintered body is used, forexample, for materials for IC substrates and electronic device packagesas well as heat-sinking materials for semiconductor devices.

Several methods have been suggested in relation to the method forproducing aluminum nitride, in which no alumina powder is used, andpseudoboehmite, which is one of precursors of alumina powder, is used.Japanese Patent Publication No. 04-60047 (Japanese Patent ApplicationLaid-open No. 63-139008) discloses a method wherein an aluminum nitridepowder, in which particles of not less than 1 μm are extremely decreasedand the particle size distribution is narrow, is produced by dispersinga pseudoboehmite powder in an acidic aqueous solution of pH 1.2 to 4.5to prepare a boehmite sol, followed by being solidified, dried, and thencalcined in a nitrogen atmosphere. In this method, the boehmite sol isgelated and solidified. Therefore, it is impossible to perform thedehydration by any simple operation such as filtration. The obtainedaluminum nitride powder is a particulate powder having fine and uniformparticle size which inherits the particle size of the boehmite highlydispersed in the acidic aqueous solution. The obtained aluminum nitridepowder is used as a raw material for any sintered body. In general, itis considered to be favorable that the powder, which is to be used forthe raw material for the sintered body, has the particulate structure inwhich the particle size is fine and uniform, for the following reason.That is, the driving force for the sintering is the decrease in thesurface energy, i.e., the decrease in the surface area. Therefore, thepowder, which has the fine and uniform particle size and which isparticulate, has the large surface area so that the surface energy isincreased. As a result, the sintering speed is increased, and the powderis easily sintered even at a low temperature. Further, it is easy toobtain the dense sintered body, because the filling density of thepowder is increased as well. It is considered that when the particulatepowder as described above is sintered, then the sintering is performeduniformly to obtain the dense sintered body, and any three-dimensionalnetwork structure is not obtained.

Japanese Patent Application Laid-open No. 63-11506 discloses a methodfor producing aluminum nitride based on the dry process in which a mixedpowder of pseudoboehmite and carbon powder is introduced into an aluminapot together with alumina balls, and the alumina pot is installed in afurnace to thermally treat the mixed powder. The aluminum nitridepowder, which is obtained by this method, is characterized in that thespecific surface area is large, and the particle size is small anduniform as compared with a powder obtained by using an ordinary aluminapowder as a raw material. The aluminum nitride powder is used as a rawmaterial for any sintered body. Also in this case, it is considered thatwhen the obtained powder is sintered, the powder is sintered uniformlyto obtain a dense sintered body, and no three-dimensional networkstructure is obtained, in the same manner as in Japanese PatentPublication No. 04-60047.

Japanese Patent Application Laid-open Nos. 04-65308, 04-59610, and04-50106 disclose a method in which an alumina powder and a carbonpowder are mixed in an aqueous dispersion medium added with asurfactant, and then a heating reaction is effected in an atmospherecontaining nitrogen to produce an aluminum nitride powder. For example,Japanese Patent Application Laid-open No. 04-65308 discloses a method inwhich an alumina powder and a carbon powder are mixed in an aqueousdispersion medium to which a nonionic surfactant is added whileadjusting pH to a range between 9 and 13 to obtain a mixed slurry, andthen acid is added to the mixed slurry to change pH to a range between 2and 6. When pH is firstly adjusted to a range between 9 and 13, thecarbon powder is highly dispersed in accordance with the action of thenonionic surfactant. Subsequently, the carbon powder is allowed to enterthe interstices of the highly dispersed alumina powder by changing PH toa range between 2 and 6. The alumina powder used in this method includesalumina of the generally commercially available grades of low sodaalumina, high purity alumina, and reactive alumina. The alumina asdescribed above is different from pseudoboehmite which is a precursorfor alumina to be obtained by the neutralization method or the ALFOLmethod. Each of the low soda alumina and the reactive alumina is analumina powder obtained by calcinating gibbsite as a precursor producedby the Bayer method. The high purity alumina is an alumina powderobtained by the pyrolysis of alum, the ALFOL method, or the gas phasehydrolysis method (see, for example, “Alumina Powder and Their Prices”,Kazuhiko Nakano, Ceramics, 36 (2001), No. 4, pp. 248-253).

It is described that the method as described above makes it possible toobtain aluminum nitride which includes a small amount of coagulatedparticles, which has a sharp particle size distribution, and which hashigh sintering performance, by enhancing the dispersibility of both ofthe alumina powder and the carbon powder by changing pH of the aqueoussolution while using the surfactant. However, each of the low sodaalumina, the high purity alumina, and the reactive alumina is the finelyparticulate powder with the uniform particle size. Therefore, it isconsidered that any aluminum nitride, which has the three-dimensionalnetwork structure as in the present invention, is not obtained even whenthe alumina as described above is sintered.

In recent years, the particulate aluminum nitride is expected to be usedfor a way of use as the filler for semiconductor package moldingcompound. Conventionally, spherical silica or the like has been used asthe filler for the semiconductor package molding compound. However, inorder to successfully dissipate the heat generated in the device, it isdemanded to use a filler having high thermal conductivity. Aluminumnitride is expected as a filler material to replace silica, owing to thehigh thermal conductivity thereof. On the other hand, in the case of theuse for the semiconductor package molding compound and the sensormaterial, it is desirable that the filler is constructed with a highpurity aluminum nitride. Therefore, the method, in which the carbon andthe alumina powder are dispersed by using the surfactant as describedabove, is undesirable, because the component of the surfactant remainsin the aluminum nitride product.

When the consideration is made about the aluminum nitride for the way ofuse as the filler, it is required to provide particles having largeparticle sizes with a high sphericity and a wide particle sizedistribution of not less than the micron order.

In the case of the direct nitridation described above, the high puritymetal aluminum powder is directly heated and nitrided in the nitrogengas flow. The particle size is relatively large immediately after thenitridation, because the aluminum, which is melted and fused in thenitridation step, is inevitably coagulated. Therefore, in order toobtain particles for the filler, it has been required that thecoagulation, which are obtained immediately after the nitridation, arepulverized and classified to obtain particle sizes of several to severaltens micron order. However, as a result of the pulverization, theparticle shape is angular, non-spherical, and formless. Therefore,problems arise such that the fluidity is lowered, and any impurityeasily makes contamination during the repeated pulverizing steps.

In the case of the carbothermal reduction-nitridation described above,the aluminum nitride powder, which has a high purity and a uniformparticle size, can be obtained by mixing the high purity alumina powderand the carbon powder and heating in the nitrogen atmosphere. However,this method is basically a method for producing any fine powder for thesintered body. The raw material alumina to be used usually has the sharpparticle size distribution of the submicron diameter. As a result, theproduced powder also resides in particles of about several micronshaving a sharp primary particle size distribution of the submicrondiameter. Therefore, the produced powder is not suitable for the way ofuse for the filler.

As described above, in the case of the conventional carbothermalreduction-nitridation and the direct nitridation, it has been difficultto synthesize any aluminum nitride for the filler which has both of thewide particle size distribution and the high fluidity.

Japanese Patent Application Laid-open Nos. 04-50107 and 05-270810disclose a method in which an aluminum nitride powder is granulated anddried to obtain granular aluminum nitride thereby. In the methoddescribed in each of the patent documents, for example, a sintering aidand a solvent are added to the fine aluminum nitride powder to mix themin a wet state, and the mixture is dried and granulated, for example, byusing a spray dryer. However, the obtained aluminum nitride is to beused as a raw material for a sintered body. Although the granulatedgranules have large particle sizes, the granules are merely obtained bydrying and coagulating the fine aluminum nitride powder. The thermalconductivity as the bulk is low, because the granules are not anysintered body. Further, in the patent documents described above, thealuminum nitride powder is used as the starting material, and there isno description about the way of production of the aluminum nitridepowder.

Japanese Patent Application Laid-open Nos. 04-174910 and 11-269302disclose a method in which granulated granules are heated at a hightemperature to obtain a sintered body having satisfactory thermalconductivity. In the method described in each of the patent documents, asintering aid and a forming aid are added to an aluminum nitride powderto perform the mixing in a wet state, followed by performing the spraydrying to obtain the granulated granules which are sintered at a hightemperature in nitrogen. The sintered granules are used as a filler forsemiconductor packing molding resins and heat-sinking members composedof silicone rubber. In the patent documents described above, thealuminum nitride powder is used as the starting material, and there isno description about the way of production of the aluminum nitridepowder. Further, the method described in each of the patent documentsrequires a step of producing the aluminum nitride powder, a step ofgranulating the powder to obtain large particle size granules, and astep of sintering the large particle size granules at a hightemperature. The steps, which are required to obtain the sintered body(sintered granules) from the aluminum nitride powder, are complicated,and the cost is expensive. Further, the granulated granules arethermally deposited to one another. Therefore, it is necessary to avoidsuch an inconvenience. In Japanese Patent Application Laid-open Nos.04-174910 and 11-269302 described above, a boron nitride powder is addedto granulated granules in order to avoid the thermal deposition betweenthe granulated granules. Such an operation also makes the steps to becomplicated. A magnified photograph of a spherical sintered granule ofaluminum nitride is shown in Japanese Patent Application Laid-open No.04-174910. However, according to this photograph, the obtained sinteredgranule resides in a high density particle having a dense structure,which has no three-dimensional network structure.

DISCLOSURE OF THE INVENTION

A first object of the present invention is to produce spherical aluminumnitride which is excellent in the thermal conductivity and the fluidityby the carbothermal reduction-nitridation of the precursor granulatedand dried to have a spherical form. A second object of the presentinvention is to produce aluminum nitride particles which have a wideparticle size distribution favorable for the ways of use as a filler. Athird object of the present invention is to produce aluminum nitridewithout using any organic solvent which is undesirable in view of theenvironment and any surfactant as a dispersing agent which lowers theproduct purity.

According to a first aspect of the present invention, there is provideda method for producing aluminum nitride comprising:

-   -   preparing a slurry containing pseudoboehmite and carbon;    -   filtrating and dehydrating the prepared slurry to obtain a        precursor; and    -   calcinating the obtained precursor in an atmosphere containing        nitrogen.

In the method of the present invention, the slurry, which contains thefinely particulate pseudoboehmite and the carbon source, is granulatedto have large particle sizes with ease by selecting the condition of theslurry. The carbon source, which serves as a reducing agent, isdispersed in the precursor granulated to have the objective particlesize. Therefore, the carbothermal reduction-nitridation of the precursoris advanced uniformly. Therefore, the aluminum nitride, which has adesired particle size distribution and a desired shape, can be producedin accordance with the convenient steps.

In order to improve the dispersibility of the carbon source, it ispreferable that the slurry containing the pseudoboehmite and the carbonis prepared by synthesizing the pseudoboehmite in a solution containingthe carbon. In particular, it is preferable that the pseudoboehmite issynthesized by a neutralization reaction in which an aluminum source isat least one of aluminum salt and aluminic acid. During the synthesis,the carbon powder or the like, which serves as a reducing agent, isdispersed while adjusting pH of the aqueous solution in a range of 8.5to 9.5, and then the neutralization reaction is subsequently performedwhile adjusting pH in a range of 8.5 to 9.5. Thus, it is possible toform the precursor which includes the carbon source more uniformly andwhich is composed of fibrous pseudoboehmite.

In the method of the present invention, it is unnecessary to use anysurfactant in order to disperse the carbon source and the pseudoboehmitein the aqueous solution. Therefore, the obtained aluminum nitride has ahigh purity, which is preferably usable, for example, for the way of usefor package molding compound for electronic devices. In the method ofthe present invention, it is also unnecessary to use any organic solventin order to disperse the carbon source such as carbon black which ishydrophobic.

It is preferable that the temperature of the aqueous solution ismaintained in a range of 55 to 75° C. during the synthesis of thepseudoboehmite in order to facilitate the formation of the fibrouspseudoboehmite. It is preferable that the obtained slurry, whichcontains the precursor, is filtrated and dehydrated to obtain theprecursor, and the precursor is granulated to have an average particlesize of 5 to 1,000 μm so that the spherical granulated precursor isobtained. When the granulated precursor is calcinated in a nitrogenatmosphere in accordance with the method of the present invention, it ispossible to produce the aluminum nitride particle which is spherical andwhich has the three-dimensional network structure. The sphericalaluminum nitride particle provides the high fluidity, and thethree-dimensional network structure provides the high thermalconductivity. Therefore, the aluminum nitride is preferably usable forthe way of use for the filler.

It is appropriate that the temperature for calcinating particles is sucha temperature, for example, 1,300 to 1,600° C. that the carbothermalreduction-nitridation occurs to obtain the aluminum nitride particles.When the calcination is performed at a temperature of the degree asdescribed above, the aluminum nitride particle, which has thethree-dimensional network structure, is obtained. Therefore, it isunnecessary to perform the calcination at a high temperature at whichthe sintering is excessively caused. It is possible to avoid any thermaldeposition of aluminum nitride particles.

The granulation operation can be carried out with a spray dryer. It ispreferable that the granulation is performed so that 20 to 50% of allgranules exist within a range of ±20% of the average particle size ofthe granulated precursor.

According to a second aspect of the present invention, there areprovided spherical aluminum nitride particles produced by the productionmethod of the present invention, wherein a three-dimensional networkstructure is provided in the particle. It is considered that thethree-dimensional network structure of the aluminum nitride particlesconstitutes the heat-sinking passages. When the aluminum nitrideparticles as described above are used, it is possible to produce afiller which is excellent in the thermal conductivity. It is preferablethat the aspect ratio of the fibrous aluminum nitride for constructingthe network is not less than 7 in the three-dimensional networkstructure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows particle size distributions of aluminum nitride particlesobtained in embodiments of the present invention.

FIG. 2 shows a scanning type electron microscope photograph illustratingan internal structure of the aluminum nitride particle obtained in theembodiment of the present invention.

FIG. 3 shows a scanning type electron microscope photograph illustratingan internal structure of the commercially available aluminum nitrideparticles for any sintered body.

FIG. 4(A) shows a schematic structure of fibrous pseudoboehmite, FIG.4(B) schematically shows a magnified view illustrating each of fiberscomposed of the mutually bonded wrinkled sheet, and FIG. 4(C)conceptually shows a situation in which carbon black particles areadhered to the wrinkled sheet.

BEST MODE FOR CARRYING OUT THE INVENTION

Pseudoboehmite

The pseudoboehmite refers to a material having low crystallinity asboehmite which is an aluminum oxyhydroxide. The pseudoboehmite is analumina hydrate having extra water molecules in the crystals, which isrepresented by Al₂O₃.xH₂O, wherein x is not less than 1 and less than 2.The X-ray diffraction index of boehmite is described in ASTM Card No.5-0190. It is preferable to use pseudoboehmite having a (020) planedistance of 6.4 to 6.7 angstroms. It is preferable that the crystallitediameter determined from the width of the (020) diffraction peak is 2.0to 5.0 nm, and the crystallite diameter determined from the width of the(120) diffraction peak is 2.5 to 7.0 nm. The pseudoboehmite is producedby the neutralization method with aluminum salt and the hydrolysismethod with aluminum alkoxide. The pseudoboehmite has such a featurethat the dispersibility is high in an acidic aqueous solution. Theparticulate structure of boehmite is classified into the acicular, theisometrical, the intermediate therebetween, and the wrinkled sheet. Inthe wrinkled sheet, fine and plate-shaped boehmite primary particlesform fibrous aggregate bonded face to face and edge to edge. FIG. 4(A)shows a schematic structure of fibrous pseudoboehmite, and FIG. 4(B)schematically shows a magnified view illustrating each of fiberscomposed of mutually bonded wrinkled sheets. When the wrinkledsheet-shaped particles are developed, then the agglomerated structure ofparticles is coarse, and the bulk density is lowered (for example,Journal of the Ceramic Society of Japan, 107 (4), 359-364 (1999)). It ispreferable that the pseudoboehmite, which is usable for the method ofthe present invention, is fibrous pseudoboehmite of the wrinkled sheettype having an aspect ratio of not less than 10 in order to provide thethree-dimensional network structure for the aluminum nitride. It isespecially preferable that the average aspect ratio is not less than 50.

It is preferable that the pseudoboehmite to be used for the presentinvention is synthesized in accordance with the neutralization reactionby using an aluminum source of at least one of aluminum salt andaluminic acid. Those preferably usable as the aluminum salt includeaqueous solutions of acidic aluminum salts such as aluminum sulfate andaluminum chloride. In this procedure, it is preferable that the aluminumconcentration of the acidic aluminum aqueous solution is 0.5 to 3 mol/L.The aluminum concentration of the aqueous aluminic acid solution ispreferably 0.3 to 6 mol/L and especially preferably 0.5 to 4 mol/L. Themolar ratio of alkali/aluminum is preferably 1 to 3 and especiallypreferably 1.5 to 2.8. The pseudoboehmite powder can be produced inaccordance with the neutralization reaction of aluminum salt andaluminic acid. It is desirable that the temperature of the aqueoussolution is controlled during the synthesis of the pseudoboehmite. Whenthe synthesis temperature is 30 to 80° C., especially 55 to 75° C., thefibrous precursor is produced with ease. If the temperature is lowerthan 30° C., then the structure of the precursor is not fibrous, thestructure is approximately particulate, and unreacted matters of acidand alkali tend to remain. The properties of the precursor after thesynthesis are unstable, which tend to be changed in a time-dependentmanner. If the temperature is higher than 80° C., it is necessary torespond to the high temperature in relation to the equipment disposed atthe lower stages to be used, for example, for the extraction or thedrawing of the slurry after the synthesis, the filtration, and thedehydration.

Carbon black is preferably usable as the carbon source. The averageparticle size of the carbon black is preferably 1 to 1,000 nm andespecially preferably 1 to 100 nm. The amount of use of the carbon blackis preferably an amount sufficient to reduce the pseudoboehmite. Thecarbon black is nano-scale fine particles composed of amorphouscarbonaceous matter of not less than about 95%. The carbon black isobtained by treating hydrocarbon in a combustion gas atmosphere at 1,300to 1,700° C. The size of the fine particle (basic unit) is about severalto several hundreds nm, i.e., the size is fine and minute. However, ingeneral, the fine particles are coagulated to construct themicrostructure of several to several tens microns called “structure”. Asfor the carbon source other than the carbon black, it is also allowableto use an organic compound such as hydrocarbon which is converted intocarbon by performing the heat treatment in a nitrogen atmosphere. It ispreferable that the carbon source is allowed to co-exist in the reactionsolution when the pseudoboehmite is synthesized in accordance with theneutralization reaction of aluminum salt and/or aluminic acid.Accordingly, the carbon source and the pseudoboehmite are uniformlydispersed, and the slurry is easily filtrated and dehydrated as well.

In the method of the present invention, the slurry, which contains thepseudoboehmite and the carbon, can be prepared as follows. Apredetermined amount of the carbon black is introduced into deionizedwater having pH adjusted between 8.5 and 9.5, and the carbon black isdispersed for a predetermined period of time. After that, at least oneof aluminum salt and aluminic acid as the aluminum source is fed intothe aqueous solution while adjusting the flow rate so that pH is between8.5 and 9.5 to perform the neutralization reaction. As a result of theneutralization reaction, it is possible to form the slurry in which thepseudoboehmite having the fibrous aggregated structure is dispersed. Thereason, why pH of the aqueous solution is adjusted between 8.5 and 9.5,is that the feature of the satisfactory dispersibility of the carbonblack in the alkali region is intended to be effectively utilized. Inthis pH region, it is possible to satisfactorily disperse the carbonblack in the aqueous solution even when no organic solvent is used.Further, when the neutralization reaction is performed in this pHregion, it is possible to obtain the slurry which contains thepseudoboehmite having the fibrous aggregated structure. The slurry canbe used to form the precursor composed of the pseudoboehmite having thefibrous structure in which the carbon black is uniformly dispersed. Itis considered that the carbon black is adhered to the wrinkled sheetsfor constructing the fibrous pseudoboehmite as shown in FIG. 4(C). If pHis less than 8.5, it is feared that the dispersibility of the carbonblack may be lowered, and any sedimentation and/or any separation mayarise. On the contrary, the dispersibility of the pseudoboehmite isenhanced, and the fibrous structure is insufficiently formed.Simultaneously, the dispersed particles clog up and close the meshes ofthe filter cloth, and the filterability of the slurry is lowered. On theother hand, if pH exceeds 9.5, then the pseudoboehmite is not generated,the bayerite tends to be formed, and the solubility of thepseudoboehmite is increased. For example, the agitation with mixingimpellers or the ultrasonic dispersion may be adopted as the method fordispersing the carbon black added into the deionized water having pHadjusted between 8.5 and 9.5. As for other components, it is alsoallowable to add, for example, any facilitating agent for nitridationsuch as calcium carbonate and yttrium oxide to the deionized water.

The specified method of the present invention is different in thefollowing point from the conventional method based on the use of thesurfactant as described above (Japanese Patent Application Laid-openNos. 04-65308, 04-59610, and 04-50106). That is, in the method of thepresent invention, the pseudoboehmite is not dispersed in the solution.The pseudoboehmite having the fibrous structure is formed while highlydispersing the carbon black in the state in which the solution iscontrolled to be weakly alkaline.

According to the method of the present invention, the slurry, which isprepared as described above, is filtrated, dehydrated, washed, andformed into a cake which is prepared into the precursor. The filtration,the dehydration, and the washing can be performed in accordance with thestandard method based on the use of the belt filter and the filterpress. The precursor is calcinated in a non-oxidizing atmospherecontaining a nitrogen source such as nitrogen gas and ammonia gas. Inorder to provide the non-oxidizing atmosphere, it is preferable that theprecursor contains the carbon source in such an amount that the oxidesuch as alumina can be sufficiently reduced. The calcination temperatureis preferably 1,200 to 1,700° C. and especially preferably 1,300 to1,600° C. The aluminum nitride, which is formed as described above, hasthe three-dimensional network structure.

In order to produce the spherical aluminum nitride, the followingprocedure is preferably adopted. That is, the slurry, which contains thepseudoboehmite and the carbon source, is prepared, followed by beingfiltrated and dehydrated to obtain the precursor. The precursor isgranulated to have an average particle size of 5 to 1,000 μm to obtainthe spherical granulated precursor. The spherical granulated precursorpreferably has an average particle size of 10 to 500 μm and especiallypreferably 20 to 250 μm. When 20 to 50% of all granules exist within arange of ±20% of the average particle size of the granulated precursor,it is possible to obtain the aluminum nitride particles having a wideparticle size distribution. Those usable as the method for thegranulation include, for example, the mixing granulation method such asthe rolling granulation and the fluidized bed granulation, the forciblegranulation method such as the compression molding and the extrusiongranulation, and the heat-based granulation method such as the meltinggranulation and the spraying granulation. In particular, the spraydrying is preferably used. The spray drying is the method for forminggranules belonging to the spray drying granulation method, in which theliquid slurry is finely sprayed in a high temperature gas flow toinstantaneously perform the drying.

The aluminum nitride particle, which preferably has an average particlesize of 5 to 1,000 μm, especially preferably 10 to 500 μm, and moreespecially preferably 20 to 250 μm, can be obtained by calcinating thegranulated precursor in the non-oxidizing atmosphere containing thenitrogen source such as nitrogen gas and ammonia gas. It is possible toobtain the wide particle size distribution in which 20 to 50% of allparticles exist within the range of ±20% of the average particle size.The particulate aluminum nitride is substantially spherical, which isusable, for example, for producing the high density formed product andthe filling material for heat sinking. In particular, the aluminumnitride is most suitable for the filler for the semiconductor packagemolding compound.

The precursor is subjected to the carbothermal reduction-nitridation bycalcinating the precursor in the non-oxidizing atmosphere containing thenitrogen source. As a result of the carbothermal reduction-nitridation,the three-dimensional network structure of the aluminum nitride isformed. The three-dimensional network structure functions as thepassages for the heat during the heat conduction. Therefore, when thecalcination is performed at a temperature of such an extent that thealuminum nitride is obtained by the carbothermal reduction-nitridation,the particles having the high thermal conductivity are obtained.Therefore, it is possible to avoid any thermal deposition of particles,which would be otherwise caused by any excessive sintering. The reason,why the aluminum nitride having the three-dimensional network structureis formed when the fibrous pseudoboehmite is used as the precursor, isunknown. Any detailed mechanism has not been proved yet.

The width (diameter) of the fiber (fibrous aluminum nitride) forconstructing the three-dimensional network structure of the obtainedaluminum nitride is about 0.6 to 0.8 μm in average. The length of thefiber for constructing the network is approximately not less than 5 μm.In order to provide the sufficient thermal conductivity, it is desirablethat the aspect ratio of the aluminum nitride fiber for constructing thenetwork is not less than 7 in average.

EXAMPLES

The present invention will be explained in further detail below withreference to Examples. However, the present invention is not limited toExamples.

Example 1

4.4 kg of carbon black having an average particle size of 20 nm as acarbon source and 0.5 kg of calcium carbonate as a nitriding aid wereintroduced while agitating 315 L of ion exchange water introduced into areaction vessel by using an agitator, followed by being heated to 55° C.while adjusting pH at 8.5. 62 L of aluminum sulfate aqueous solutionhaving an aluminum concentration of 1 mole/L and 123 L of sodiumaluminate aqueous solution were heated to 55° C. in separate vesselsrespectively. After that, the aluminum sulfate aqueous solution and thesodium aluminate aqueous solution were simultaneously introduced intothe reaction vessel, followed by being agitated. The aluminum sulfateaqueous solution was fed at a constant feeding rate (3.1 L/minute) sothat the feeding time was 20 minutes. During this process, the feedingrate of the sodium aluminate aqueous solution was adjusted so that pHwas 8.5 in the reaction vessel. A slurry, which was synthesized asdescribed above, was in an amount of 500 L. The aluminum concentrationwas 0.37 mole per 1 L of the slurry.

1 L of the synthesized slurry was sampled, and the filterability wasconfirmed in accordance with the following method. Filter paper(produced by Advantec, No. 3) was placed on a Buchner funnel having adiameter of 110 mm, and the slurry was introduced while effecting thesuction with a pump. The time, which was required from the start of theintroduction of the slurry to the appearance of cracks in the cake afterthe disappearance of the filtrate, was measured. As a result, the timewas about 1 minute which was short. Thus, the satisfactory filterabilitywas exhibited. The components for constructing the cake were analyzed tobe carbon and pseudoboehmite. In particular, it was clarified by theX-ray diffraction measurement that the component was pseudoboehmite.Subsequently, the pseudoboehmite was observed by using a transmissionelectron microscope (TEM). Ten samples were randomly extracted from theparticles existing in the image field. The lengths of the minor axis andthe major axis of each of the particles were measured. As a result, theform was fibrous, and the aspect ratio was 12 to 120. The aspect ratiowas 65 in average.

The synthesized slurry was fed to a belt filter, followed by beingfiltrated and dehydrated to obtain a cake. Ion exchange water wassprayed from a position over the filter cloth during the filtration andthe dehydration to simultaneously wash out the salt content in the cake.The water content of the obtained cake was measured while effecting theheating to 130° C. by using an infrared moisture meter. As a result, thewater content was about 80%. Ion exchange water was added to the cake torepulp the cake while performing the agitation, and the slurry having asolid concentration of about 10% was prepared. The slurry was fed into aspray dryer to perform the granulation and the drying by spraying theslurry from an atomizer into the drying chamber. The granulatedprecursor (aluminum nitride precursor) was obtained as a mixture of thepseudoboehmite and the carbon black. The precursor had an averageparticle size of 38 μm, and the number of granules included within ±20%of the average particle size was 40% of the total. The average particlesize was measured by using a laser analysis type particle sizedistribution measuring apparatus (LA-500) produced by Horiba, Ltd.

The precursor was calcinated at 1,450° C. for 5 hours in a nitrogenatmosphere, and then a decarbonizing treatment was performed at 700° C.for 3 hours to obtain aluminum nitride particles. The obtained aluminumnitride had an average particle size of 39 μm, and the number ofparticles included within ±20% of the average particle size was 41% ofthe total. FIG. 1 shows a particle size distribution of aluminum nitrideparticles. According to FIG. 1, it is appreciated that the aluminumnitride particles, which are obtained in accordance with the method ofthe present invention, have the wide particle size distribution. Theshape was investigated by using a scanning type electron microscope. Asa result, it was confirmed that the shape was a clear spherical shape.Further, it was confirmed that the three-dimensional network structurewas provided. The structure is shown in FIG. 2. The widths (diameters)of the aluminum nitride fiber for constructing the network structurewere about 0.2 μm at thin portions and not less than about 1 μm at thickportions. In average, the diameter was about 0.6 to 0.8 μm. Almost allof the lengths of the fibers for constructing the network were not lessthan 5 μm, although the lengths were about 2 μm at short branchportions. Therefore, it is affirmed that the aspect ratio of thealuminum nitride fiber for constructing the network is 7.1 in average.

Example 2

4.4 kg of carbon black having an average particle size of 20 nm as acarbon source and 0.5 kg of calcium carbonate as a nitriding aid wereintroduced while agitating 315 L of ion exchange water introduced into areaction vessel by using an agitator, followed by being heated to 60° C.while adjusting pH at 9.0. 62 L of aluminum sulfate aqueous solutionhaving an aluminum concentration of 1 mole/L and 123 L of sodiumaluminate aqueous solution were heated to 60° C. in separate vesselsrespectively. After that, the aluminum sulfate aqueous solution and thesodium aluminate aqueous solution were simultaneously introduced intothe reaction vessel, followed by being agitated. The aluminum sulfateaqueous solution was fed at a constant feeding rate (3.1 L/minute) sothat the feeding time was 20 minutes. During this process, the feedingrate of the sodium aluminate aqueous solution was adjusted so that pHwas 9.0 in the reaction vessel. A slurry, which was synthesized asdescribed above, was in an amount of 500 L. The aluminum concentrationwas 0.37 mole per 1 L of the slurry.

1 L of the synthesized slurry was sampled, and the filterability wasconfirmed in accordance with the same method as that used in Example 1.As a result, the time, which was required from the start of theintroduction of the slurry to the appearance of cracks in the cake afterthe disappearance of the filtrate, was about 1 minute. Thus, thesatisfactory filterability was exhibited. The components forconstructing the cake were analyzed to be carbon and pseudoboehmite. Inparticular, it was clarified by the X-ray diffraction measurement thatthe component was pseudoboehmite. Subsequently, the pseudoboehmite wasobserved by using a transmission electron microscope (TEM) in the samemanner as in Example 1. Ten samples were randomly extracted from theparticles existing in the image field. The lengths of the minor axis andthe major axis of each of the particles were measured. As a result, thefollowing fact was revealed. That is, the form was fibrous, the aspectratio was 21 to 100, and the average value was 68.

The synthesized slurry was filtrated, dehydrated, granulated, and driedin accordance with the same method as that used in Example 1 to obtain aprecursor as a mixture of the pseudoboehmite and the carbon black. Thegranulated precursor had an average particle size of 76 μm, and thenumber of granules included within ±20% of the average particle size was48% of the total.

The precursor was calcinated at 1,450° C. for 5 hours in a nitrogenatmosphere, and then a decarbonizing treatment was performed at 700° C.for 3 hours to obtain aluminum nitride particles. The obtained aluminumnitride had an average particle size of 77 μm, and the number ofparticles included within ±20% of the average particle size was 49% ofthe total. FIG. 1 also shows a particle size distribution in this case.The shape was investigated by using a scanning type electron microscope.As a result, it was confirmed that the shape was a nice spherical shape.Further, it was confirmed that the three-dimensional network structurewas provided in the same manner as in Example 1.

Example 3

4.7 kg of carbon black having an average particle size of 20 nm as acarbon source and 0.35 kg of yttrium oxide as a nitriding aid wereintroduced while agitating 312 L of ion exchange water introduced into areaction vessel by using an agitator, followed by being heated at 60° C.while adjusting pH at 9.5. 37 L of aluminum sulfate aqueous solutionhaving an aluminum concentration of 2 mole/L and 150 L of sodiumaluminate aqueous solution were heated to 60° C. in separate vesselsrespectively. After that, the aluminum sulfate aqueous solution and thesodium aluminate aqueous solution were simultaneously introduced intothe reaction vessel, followed by being agitated. The aluminum sulfateaqueous solution was fed at a constant feeding rate (1.9 L/minute) sothat the feeding time was 20 minutes. During this process, the feedingrate of the sodium aluminate aqueous solution was adjusted so that pHwas 9.5 in the reaction vessel. A slurry, which was synthesized asdescribed above, was in an amount of 500 L. The aluminum concentrationwas 0.45 mole per 1 L of the slurry.

1 L of the synthesized slurry was sampled, and the filterability wasconfirmed in accordance with the same method as that used in Example 1.As a result, the time, which was required from the start of theintroduction of the slurry to the appearance of cracks in the cake afterthe disappearance of the filtrate, was about 1 minute. Thus, thesatisfactory filterability was exhibited. The synthesized slurry wasfiltrated and dehydrated in accordance with the same method as that usedin Example 1 to obtain the cake. The components for constructing thecake were analyzed to be carbon and pseudoboehmite. In particular, itwas clarified by the X-ray diffraction measurement that the componentwas pseudoboehmite. The pseudoboehmite was observed by using TEM in thesame manner as in Example 1. As a result, it was revealed that the rangeof the aspect ratio and the average value thereof were provided as shownin Table 1. In Table 1, the aspect ratios and the average values thereofof the pseudoboehmites obtained in Examples 1 and 2 are also shown incombination. TABLE 1 Aspect Average value Example 1 12 to 120 65 Example2 21 to 100 59 Example 3 14 to 145 68

Ion exchange water was added to the cake to repulp the cake whileperforming the agitation, and the slurry having a solid concentration ofabout 1% was prepared. The slurry was fed into a spray dryer to performthe granulation and the drying by spraying the slurry from an atomizerinto the drying chamber. The granulated precursor was obtained as amixture of the pseudoboehmite and the carbon black. The precursor had anaverage particle size of 21 μm, and the number of granules includedwithin ±20% of the average particle size was 30% of the total.

The precursor was calcinated at 1,450° C. for 5 hours in a nitrogenatmosphere, and then a decarbonizing treatment was performed at 700° C.for 3 hours to obtain aluminum nitride particle. The obtained aluminumnitride had an average particle size of 21 μm, and the number ofparticles included within ±20% of the average particle size was 30% ofthe total. FIG. 1 also shows a particle size distribution in this case.The shape was investigated by using a scanning type electron microscope.As a result, it was confirmed that the shape was a nice spherical shape.Further, it was confirmed that the three-dimensional network structurewas provided in the same manner as in Example 1.

Example 4

The spherical particles (measured value of void fraction ε=0.40) havingan average particle size of 77 μm obtained in Example 2 is blended withthe spherical particles (measured value of void fraction ε=0.43) havingan average particle size of 21 μm obtained in Example 3 to prepare afiller material. The void fraction of the packed bed composed of the twoof the large and small particle components was calculated. As a result,it was revealed that the minimum void fraction εmin=0.3322 was obtainedat a mixing partial ratio Sv=0.25 of the small particles having theaverage particle size of 21 μm. Therefore, the two types of theparticles were blended at a ratio of 0.75 of the particles having theaverage particle size of 77 μm and a ratio of 0.25 of the particleshaving the average particle size of 21 μm to obtain a filler material of80 parts by weight. The material was blended with 20 parts by weight ofnovolac epoxy resin. The coefficient of thermal conductivity of theresin was measured by using a thermal constant-measuring apparatus basedon the laser flash method (produced by Rigaku Corporation). For thepurpose of comparison, the coefficient of thermal conductivity wasmeasured in the same manner as described above for a case in which 80parts by weight of fused silica as a conventional filler material wasblended as a filler with 20 parts by weight of novolac epoxy resin.Results of the measurement are shown in Table 2. It was confirmed thatthe filler, which was formed of the aluminum nitride of the presentinvention, had the high coefficient of thermal conductivity of the resinand the filler was excellent in the heat-sinking performance as comparedwith the filler composed of the conventional fused silica. The fillermaterial of the present invention is spherical, and the filler materialhas the high fluidity. Therefore, it is possible to improve thecoefficient of thermal conductivity by effecting the filling at a higherdensity. In order to prevent the filler from being hydrolyzed, it isalso preferable that a coating is further applied to the obtained fillerto avoid any hydrolysis. TABLE 2 Coefficient of thermal conductivity(W/mk) Aluminum nitride 4.5 Fused silica 0.5

INDUSTRIAL APPLICABILITY

According to the method for producing the aluminum nitride of thepresent invention, it is possible to produce the spherical aluminumnitride particle which is excellent in the thermal conductivity and thefluidity, in accordance with the simple steps at low cost. In the methodfor producing the aluminum nitride of the present invention, the feedingof the carbon source, the aluminum salt, and the aluminic acid and theextraction or drawing of the formed slurry are continuously performed inthe neutralization reaction step, or the formed slurry is once receivedby providing the tank at the downstream of the neutralization reactiontank, and thus the aluminum nitride precursor can be continuouslyproduced. Therefore, the method for producing the aluminum nitride ofthe present invention is excellent in the productivity. In the methodfor producing the aluminum nitride of the present invention, it isunnecessary to use any surfactant and any organic solvent in order todisperse the carbon source. Therefore, the method for producing thealuminum nitride of the present invention is excellent in view of theenvironment as well. The high purity aluminum nitride is obtained.

The aluminum nitride produced by the method of the present invention hasthe spherical form excellent in the fluidity, which has the wideparticle size distribution and which has the three-dimensional networkstructure. Therefore, the aluminum nitride produced by the method of thepresent invention is excellent in the thermal conductivity. Therefore,the aluminum nitride produced by the method of the present invention ispreferably usable as the filler for electronic materials and the porousmember, for example, for sensor materials and adsorbing agents.

1. A method for producing aluminum nitride comprising: preparing aslurry containing pseudoboehmite and carbon; filtrating and dehydratingthe prepared slurry to obtain a precursor; and calcinating the obtainedprecursor in an atmosphere containing nitrogen.
 2. The method forproducing the aluminum nitride according to claim 1, wherein thepseudoboehmite is fibrous pseudoboehmite having an aspect ratio of notless than
 10. 3. The method for producing the aluminum nitride accordingto claim 1, wherein the slurry containing the pseudoboehmite and thecarbon is prepared by synthesizing the pseudoboehmite in a solutioncontaining the carbon.
 4. The method for producing the aluminum nitrideaccording to claim 3, wherein the pseudoboehmite is synthesized by aneutralization reaction in which an aluminum source is at least one ofaluminum salt and aluminic acid.
 5. The method for producing thealuminum nitride according to claim 3, wherein a carbon source isdispersed in an aqueous solution having pH of a range of 8.5 to 9.5 toprepare an aqueous solution containing the carbon, and thepseudoboehmite is synthesized while maintaining pH of the aqueoussolution in a range of 8.5 to 9.5.
 6. The method for producing thealuminum nitride according to claim 5, wherein the pseudoboehmite issynthesized while maintaining a temperature of the aqueous solution in arange of 55 to 75° C.
 7. The method for producing the aluminum nitrideaccording to claim 1, further comprising granulating the prepared slurryto have an average particle size of 5 to 1,000 μm.
 8. The method forproducing the aluminum nitride according to claim 7, wherein thegranulation is performed with a spray dryer.
 9. The method for producingthe aluminum nitride according to claim 7, wherein the granulation isperformed so that 20 to 50% of all granules exist within a range of ±20%of the average particle size of the precursor.
 10. Aluminum nitrideparticles produced by the production method as defined in claim 1,wherein a three-dimensional network structure is provided in theparticle.
 11. The aluminum nitride particles according to claim 10,wherein fiber, which constitutes a network of the three-dimensionalnetwork structure, has an aspect ratio of not less than
 7. 12. Thealuminum nitride particles according to claim 10, which are used as afiller for a package molding compound.